Reactions carbene transfer

Various carbene-transfer reactions can be used with both electron-rich and electron-poor alkynes to make fluorinated cyclopropenes [9. 13, 79, 80, 81, 82] (Table 4). Haloacetylenes are too thermally unstable for most cycloaddition conditions, and simple fluorinated cyclopropenes are made by other methods [32, 45, 83, 84] (equations 30-32). 

Carbene transfer reactions with participation of organometallic compounds bearing heterocyclic substituents 99CSR315. 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

The carbene transfer reaction from silver NHC complexes has developed into a standard procedure for the synthesis of NHC complexes. This versatile procedure was introduced by Lin et al. in 1998 [102]. It is based on the preparation of silver NHC complexes which are obtained in good yield by the in situ deprotonation of azolium salts with silver oxide (Fig. 9). Depending on the counter ions present in the azolium salt and the steric demand of the N,N -substituents, complexes 25a-25c... 

Mononuclear oxazolines have been less studied. Moderate enantioselectivities (up to 60%) were obtained using PyrOx as ligands for the copper(l)-catalyzed carbene-transfer reaction of ethyl diazoacetate to styrene." " " However, the diastereo-selectivities (cis/trans) in these reactions were generally poor. 

Diazocarbonyl compounds are optimum for these transformations, and they may be readily prepared by a variety of methods. The use of iodonium ylides (17) has also been developed, " but they exhibit no obvious advantage for selectivity in carbene-transfer reactions. Enantioselection is much higher with diazoacetates than with diazoacetoacetates (18). 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

In connection with these catalytic cyclopropanation reactions, it should be mentioned that the isolable ruthenium-carbene complex 162, which is obtained from 19, [RuCMp-cymene)]2 and 2,6-bis(4-isopropyl-l,3-oxazolin-2-yl)pyridine, reacts with styrene at elevated temperature in a carbene transfer reaction83 (equation 41). Since complex 162 is also catalytically active for (alkoxycarbonyl)carbene transfer to olefins, this reaction represents one of the few connecting links between catalytic and stoichiometric carbene transfer reactions of metal-carbene complexes. 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

In another copper-mediated carbene transfer reaction, diazoester 222 has been decomposed in the presence of bis(triethylsilyl- or -germyl)mercury (equation 72) it was assumed that the obtained ketenes 223 result from the insertion of ethoxycarbonyl(trimethylsilyl)carbene into a Hg-element bond followed by a cyclic fragmentation process110. 

Scheme 9.13 Iron-catalyzed carbene transfer reactions.

Nevertheless, Hossain and coworkers also investigated the carbene transfer reaction using a chiral bimetallic iron-carbene complex, which also exhibits excellent... 

Scheme 9.17 Diastereoselective iron-catalyzed carbene transfer reaction.

Scheme 9.19 Benchmark reactions in carbene transfer reactions to alkenes.

Many oxene, nitrene and carbene transfer reactions using well-designed chiral first- and second-generation metallosalen complexes as catalysts. 

Apart from this feature there are many similarities between ylides and carbene complexes, primarily among the structural criteria. The carbene carbon may be, but not necessarily, in a planar configuration, and the M—C bonding indicates some multiple bonding character just as in most of the ylides. On the other hand, carbene transfer reactions have been observed with ylides [e.g., Eq. (36)3, indicating that the carbene complex formalism can, indeed, be successfully applied with ylides. There is hope, therefore, for a fruitful symbiosis of ylide and carbene complex chemistry, which may soon become complementary as more data become available from this new area of transition metal chemistry. 

The two carbene units can be embedded in a (macro)cyclic ring system known as a cyclophane. A standard procedure for the synthesis of such a system starts with a,a -dibromoxylene and potassium imidazolide [368]. Cychsation can be achieved by reacting the bis-imidazole compound with a second equivalent of a,a -dibromoxylene (see Figure 3.116). The cyclic bis-imidazolium cyclophane can then be reacted with paUadium(II) acetate to form the palladium complex [369,370]. The silver(I) and gold(I) complexes are accessible from the reaction with silver(I) oxide [371] and the usual carbene transfer reaction to gold(I) [372]. 

Although silver-mediated oxidative decarboxylation was known for years, its application in synthetic chemistry was very limited (107-110). Systematic studies of this chemistry and other silver-mediated oxidation chemistry in homogeneous solution is rare. This result may be due to the inherent difficulties in working with silver catalysts, which include sensitivity to ligand environment and relative inertness toward oxidation. However, these drawbacks may be overcome with carefully tuned reaction conditions and/or supporting ligand systems. Some of the recent successes with silver nitrene and carbene-transfer reactions will be discussed in detail in Sections VI and Vll. 

---